1. Field of the Invention
The present invention relates to a minute-particle iron oxide red pigment slurry used to improve a contrast of a red phosphor layer on a picture screen of a color cathode ray tube, and a manufacturing method thereof.
2. Description of the Related Art
It is known that a pigmented phosphor layer is used to improve a contrast of a picture screen of a color cathode ray tube. In this case, there is used a pigment which has high reflectance to light having the same wavelength as that of emission spectrum of a main light emitting region of the phosphor layer and low reflectance to light having the same wavelength as that of emission spectrum of the other region. Therefore, even if light component having a wavelength which is different from that of the emission spectrum of the main light emitting region of the phosphor layer, the light component is absorbed by the pigment in the phosphor layer. Thus, it is possible to obtain a color cathode ray tube having high contrast even under bright external light. In general, a pigment made of red iron oxide is adhered to the red phosphor layer.
However, it is sometimes observed that a luminance of light emitted from the phosphor layer is lowered depending upon a size of a pigment particle adhered onto the phosphor layer or thickness of a pigment particle layer. Therefore, it is required to uniformly adhere the pigment layer onto a surface of the phosphor layer. However, it is very difficult. There is then the disadvantage that the pigment layer which is once adhered onto the phosphor layer is detached therefrom during a process of coating the phosphor layer on the cathode ray tube.
Accordingly, it is not proper that the pigmented phosphor layer is used in the color cathode ray tube in order to improve the contrast of the picture screen of the cathode ray tube.
On the other hand, it is known that red, green and blue light transmitting thin films, i.e., color filters respectively corresponding to red, green and blue phosphor regions are provided between the phosphor layer and a panel in order to improve the contrast of the picture screen of the cathode ray tube.
It is sufficient to provide the red filter having high transmittance between the red phosphor layer and the panel. However, under the condition of the color cathode ray tube manufacturing process which requires heat resistance to high temperature, e.g., 450.degree. C, it is impossible to use a red dye and a red organic pigment as a material used to form the red filter. Optimum pigments that can be used in the red filter include a minute-particle iron oxide red pigment, i.e., an iron oxide pigment which is generally called a transparent iron oxide pigment.
Such red filter is obtained by coating a pigment slurry in which a minute-particle iron oxide is highly dispersed on the panel of the color cathode ray tube. Since a red iron oxide does not transmit therethrough ultra-violet rays, it is impossible to cure a pigment slurry coated layer by poly(vinyl alcohol) (hereinafter referred to as PVA)--ammonium dichromate (hereinafter referred to as ADC) method. Therefore, a method using periodic acid is employed.
According to the PVA-ADC method, a pigment slurry added with a mixed solution of PVA and ADC is coated on an entire surface of the panel and a portion where a phosphor having a predetermined emission spectrum and a black matrix are to be formed is exposed to light through a shadow mask or an aperture grill. Then, there are removed a ADC film and a pigment slurry on a portion where phosphors having emission spectra other than the predetermined emission spectrum are to be formed.
According to the method using periodic acid, the mixed solution of PVA and ADC is coated on the entire of the panel and a portion where a phosphor having emission spectra other than a predetermined emission spectrum is to be formed is exposed to light through a shadow mask or an aperture grill. Then, the panel is developed with water and a PVA film is formed on a portion where phosphors having emission spectra other than the predetermined emission spectrum are to be formed. Therefore, a pigment slurry is coated on the entire surface of the panel and dried. Then, the PVA film is broken by using periodic acid and the panel is washed with water to remove the PVA film and the pigment slurry on the portion where the phosphors having the emission spectra other than the predetermined color is to be formed.
In the general process of manufacturing the color cathode ray tube, the red filter is formed on the black matrix. Under the condition of the cathode ray tube manufacturing process which requires heat resistance to high temperature, e.g., 450.degree. C, when carbon black composing the black matrix and iron oxide which is a component of the red filter are brought in contact with each other, there is then the possibility that the black matrix is discolored by an interaction of an oxidizing power of iron oxide and a reducing power of carbon black. Improvement in discoloration of the black matrix is demanded.